1. Technical Field
The present invention relates to fuel cells which use pure hydrogen, reformed hydrogen obtained from methanol or fossil fuels, or a liquid fuel, such as methanol, ethanol and dimethyl ether, as a fuel and uses air or oxygen as an oxidant, and more particularly relates to fuel cells which use a solid polymer as an electrolyte. More specifically, the present invention relates to a method for manufacturing a catalyst layer constituting an electrode.
2. Background Art
One of the most important factors which govern the discharge performance of solid polymer electrolyte fuel cells is the size of the reaction area at the interface of three phases formed by pores which are passages for supplying a reaction gas, a solid polymer electrolyte having proton conductivity in a moistened state, and an electrode material having electronic conductivity at the interface between a solid polymer electrolyte membrane having hydrogen ion conductivity and an electrode.
Hitherto, a layer produced by dispersing an electrode material and a polymer electrolyte in a mixed state was provided at the interface between an electrolyte membrane and a porous electrode in an attempt to increase the three-phase interface. For example, JP-B-S62-61118 and JP-B-S62-61119 use a method which comprises applying a mixture of a dispersion of polymer electrolyte and a catalyst compound on a polymer electrolyte membrane, hot-pressing the coated membrane and electrode material to join them, and then reducing the catalyst compound. They also use another method which comprises applying a mixture of a dispersion of polymer electrolyte and a catalyst compound on a polymer electrolyte membrane after reducing the catalyst compound, and hot-pressing the coated membrane and electrode material to join them.
JP-B-H02-48632 discloses a method comprising molding a porous electrode, spraying a dispersion of an ion-exchange resin on the molded electrode, and hot-pressing the electrode and the ion-exchange membrane to join them. JP-A-H03-184266 uses a method which comprises mixing a powder prepared by applying a polymer electrolyte on the surface of Nylon 12 or a styrene-based resin into an electrode, and JP-A-H03-295172 employs a method which comprises mixing a powder of polymer electrolyte into an electrode. JP-A-H05-36418 discloses a method which comprises mixing a polymer electrolyte, a catalyst, a carbon powder and a fluorocarbon resin, and forming the mixture into a film to produce an electrode.
All of the above-mentioned prior arts use alcohol solvents for the dispersion of polymer electrolyte. U.S. Pat. No. 5,211,984 reports a method which comprises preparing an inky dispersion of polymer electrolyte, a catalyst and a carbon powder using glycerin or tetrabutylammonium salt as a solvent, casting the dispersion on a polytetrafluoroethylene (hereinafter referred to as “PTFE”) film, and then transferring it onto the surface of a polymer electrolyte membrane; and a method which comprises substituting a sodium atom for a hydrogen atom of a sulfonic acid group of a polymer electrolyte membrane, applying the above inky dispersion on the surface of the membrane, and heating and drying the coat at 125° C. or higher temperature to again substitute H type for the ion-exchanging group.
In order to realize a high output density that is a characteristic of a polymer electrolyte fuel cell, it is important to form a gas channel of a reaction gas in the catalyst layer of the electrode and increase the gas permeating and diffusing performance. Therefore, it has been attempted to form the gas channel by adding a water repellent material such as a fluorocarbon resin to the electrode catalyst layer. For example, JP-A-H05-36418 disperses a PTFE powder and a carbon powder supporting a catalyst into a dispersion of polymer electrolyte and kneads them to form a catalyst layer. Further, JP-A-H04-264367 forms an electrode by using a mixed liquid prepared by mixing a carbon powder supporting a catalyst with a colloidal solution of PTFE.
Moreover, there is a proposed method in which a gas diffusion electrode for an acidic electrolyte is formed by mixing a carbon powder which received water repellent treatment using PTFE with a catalyst-supporting carbon powder (J. Electroanal. Chem., 197 (1988), p. 195). On the other hand, according to the specification of U.S. Pat. No. 5,211,984, the catalyst layer of the electrode is composed only of a polymer electrolyte, a catalyst and a carbon powder, without using a water repellent material as mentioned above.
However, when a catalyst-supporting carbon powder and a water repellent material such as fluorocarbon resin or a carbon powder which received water repellent treatment are simultaneously added to a dispersion of polymer electrolyte, much polymer electrolyte is adsorbed to the water repellent material or the carbon powder which received the water repellent treatment and consequently the degree of contact between the polymer electrolyte and the catalyst becomes insufficient and non-uniform, and thus there is a drawback that a sufficient reaction area can not be ensured at the interface between the electrode and the ion-exchange membrane or the polymer electrolyte membrane.
When an electrode is composed only of a catalyst-supporting carbon powder and a polymer electrolyte, there is a drawback that the cell voltage becomes lower or unstable at high current density due to flooding caused by the generated water.
In order to solve such problems, as disclosed in JP-A-H08-264190, the present inventor et al. tested a method in which a polymer electrolyte is made colloidal and then adsorbed to a catalyst powder. In this method, however, a noble metal catalyst present in pores smaller than the colloidal particles failed to act effectively.
Therefore, it is an object of the present invention to provide a method for producing an electrode exhibiting higher performance by effectively utilizing the merit of forming a porous catalyst layer by making a polymer electrolyte colloidal.
It is another object of the present invention to provide a method for manufacturing a polymer electrolyte fuel cell exhibiting higher performance by sufficiently and uniformly bringing a polymer electrolyte and a catalyst into contact with each other to increase the reaction area inside the electrode.
It is still another object of the present invention to provide a method for manufacturing a polymer electrolyte fuel cell exhibiting higher performance in a high current density region by forming a gas channel in a catalyst layer, without excessively coating the catalyst to increase the gas permeability of the electrode.